The present invention relates to an external member for watches, which is made of anodically oxidized aluminum alloys.
A typical conventional aluminum alloy which is used for making an external member, such as a watchcase and band, contains 0.5 to 3% of magnesium (Mg) by weight, and further contains 0.01 to 0.3% of iron (Fe) by weight and 0.1 to 0.2% of silicon (Si) by weight as impurities.
In anodic oxide film formed on such a conventional aluminum alloy as the result of anodic oxidation, impurities in the alloy, especially Fe and Si, are apt to remain therein as fine grained oxides and/or voids are formed caused by partial elution of these elements during anodic oxidation treatment.
FIG. 3 shows a schematic enlarged sectional view of an aluminum alloy watchcase wherein aluminum alloy body 3a for a watchcase is coated with an anodic oxide film 3b. Voids 3c by the elution and remained oxides 3d of impurities are formed in the anodic oxide film. Due to the voids and oxides, the film 3b assumes colors, or the surface of the film is clouded by the irregular reflection of light. In order to eliminate such a defect, the surface of the film is subject to surface smoothing processes such as polishing by buffing and chemical polishing. However, enough brightness can not be obtained. Accordingly, even if the film is dyed with dye such as golden dye, the luster thereof is far inferior to gold plating. This is caused by the color of the anode oxide film or the cloud of the film.